Method and apparatus for treating drilling mud

ABSTRACT

A method for treating drilling mud discharged from a well in an abrasive jet drilling process using ferrous abrasives to recondition the drilling mud for reuse in which the drilling mud is passed over a shale shaker to remove oversized cuttings and is then delivered into a magnetic separator in which the abrasive particles are separated from the major portion of the drilling mud. The ferrous abrasive particles from the separator are further cleaned of drilling mud in a centrifugal cleaner and dried. The dried abrasive particles are screened to remove broken abrasives, and the resultant sized abrasive particles are stored in a dry condition in a hopper from which they are introduced into drilling mud at a controlled rate and recirculated through the abrasive jet drilling process.

United States Patent 1 Arscott et al.

3,713,499 Jan. 30, 1973 METHOD AND APPARATUS FOR TREATING DRILLING MUD Inventors: Raymond L. Arseott; Eber W. Gaylord, both of Pittsburgh; Ernest A. Mori, Glenshaw; Richard A. Morris, Monroeville, all of Pa.

Assignee: Gulf Research & Development Company, Pittsburgh, Pa.

Filed: Aug. 11,1971

Appl. No.: 170,709

References Cited UNITED STATES PATENTS Primary Examiner-Ernest R. Purser Attorney-Meyer Neishloss et al.

[57] ABSTRACT A method for treating drilling mud discharged from a well in an abrasive jet drilling process using ferrous abrasives to recondition the drilling mud for reuse in which the drilling mud is passed over a shale shaker to remove ove rsized cuttings and is then delivered into a magnetic separator in which the abrasive particles are separated from the major portion of the drilling mud. The ferrous abrasive particles from the separator are further cleaned of drilling mud in a centrifugal cleaner and dried. The dried abrasive particles are screened to remove broken abrasives, and the resultant sized abrasive particles are stored in a dry condition in a hopper from which they are introduced into drilling mud at a controlled rate and recirculated through the abrasive jet drilling process.

13 Claims, 1 Drawing Figure PATENTEDJAN 30 I915 METHOD AND APPARATUS FOR TREATING DRILLING MUD This invention relates to the drilling of wells and more particularly to the treatment of drilling mud used in abrasive jet drilling to condition the mud for recirculation in the drilling operation.

In the rotary method for drilling wells, a drilling mud is circulated down through a rotating drill pipe and discharged from a drill bit at the lower end of the drill pipe. The rate of circulation of the drilling mud is such that the drilling mud picks up the cuttings cut by the drill bit from the drilled formation and carries them upwardly through theannulus surrounding the drill pipe to the surface. The drilling mud is treated at the surface to remove thecuttings and is recirculated down the well for further drilling. The drilling mud serves other purposes than transporting cuttings from the borehole. A principal purpose is to provide a hydraulic pressure in the well that exceeds the pressure of liquids in formations drilled.

In an abrasive jet drilling process recently developed to drill hard formations, abrasive particles are suspended in a drilling mud and pumped down a rotating drill pipe in a manner similar to that used in the conventional rotary drilling. The abrasive laden drilling mud is discharged at an extremely high velocity, above about 650 ft. per second, through nozzles in a drill bit at the lower end of the drill pipe and against the bottom of the borehole to cut grooves in the bottom that facilitate mechanically breaking rock extending upwardly between the grooves. Virtually all penetration of the formations drilled is accomplished in the abrasive jet drilling process by the high velocity jet streams. The circulation of the abrasive laden liquid up the annulus surrounding the drill pipe carries the cuttings to the surface where the drilling mud is treated to remove cuttings and condition the mud for reuse in the drilling.

Ferrous abrasive particles, which may be either steel or cast iron grit or shot, have been found to be highly effective in the abrasive jet drilling process. Steel shot is the preferredabrasive. The size of the abrasive particles depends on the diameter of the throat of the nozzles in the drill bit and is usually in the range of 18 80 mesh in the U. S. Sieve Series. A narrow range of particle size, for example, 30 50 mesh for use with a nozzle throat diameterof one-eighth inch, is advantageous.

Treatment of, the drilling mud used in abrasive jet drilling is substantially more difficult than the treatment of conventional drilling muds. In addition to causing severe erosion of equipment, the abrasive particles increase substantially the concentration of solids in the drilling mud and thereby make more difficult pumping the drilling mud from one vessel to another. Because of the large size and high density of the abrasive particles relative to conventional drilling mud solids, the abrasive particles settle from the drilling mud rapidly unless the drilling mud is kept moving at a high velocity. Moreover, it is necessary to remove from the drilling mud broken abrasive particles as well as oversize and undersize cuttings to maintain maximum drilling rates.

This invention resides in a method and apparatus for treating ferrous abrasive laden drilling mud discharged from the well for use in the abrasive jet drilling process in which the drilling mud discharged from the wellis screened to remove large cuttings, the ferrous abrasive particles are separated from the screened drilling mud in a magnetic separator, the abrasive particles are cleaned of adhering drilling mud, the cleaned abrasive particles are dried, and then are classified while in a dry condition to separate oversized and undersized particles from the abrasive. The abrasive particles are stored in a dry condition and fed at an accurately controlled rate into a blender for admixture with clean abrasivefree liquid to form a drilling mud and the resulting drilling mud is pumped at the high pressure needed for circulation down the well and imparting the necessary very high velocity to the liquid discharged from the drill bit nozzles.

The single FIGURE of the drawings is a diagrammatic flow sheet for treating abrasive laden drilling mud by this invention.

Referring to the drawing, a well 10 is indicated diagrammatically with a kelly 12 extending downwardly through the upper end of the well for connection to a drill pipe within the well. A mud return line 14 from well 10 discharges mud with suspended cuttings into a rotating ditch 16 which delivers the drilling mud onto a shale shaker l8. Shale shaker 18 is a vibrating screen which can be 5 to 20 mesh. The size of the openings in the shale shaker will depend on the size of the abrasive and type of drilling mud. A drilling mud that can be used in the abrasive jet process is described in U. S. Pat. No. 3,508,621 and typically contains l 6 percent clay, 0.5 5 percent plant fibers, and l 20 percent steel shot, but this invention can be used advantageously in treating any drilling mud containing ferrous abrasives. The ferrous abrasivescan be either shot or grit.

Oversize cuttings are rejected by the shale shaker and discarded from the system through a waste line 20. The drilling mud containing solid particles that pass through the screen on the shale shaker is delivered through line 22 into a magnetic separator 24 in which ferrous abrasive particles are separated from liquid and the other solid constituents of the drilling mud. Magnetic separator 24 includes a steel drum rotatably suspended in a tank into which drilling mud is delivered from the shale shaker. Abrasive particles adhere to the drum and are carried out of the tank and separated from the drum. Ferrous abrasives are discharged from separator 24 through line 26 as a concentrated slurry. The major portion of the liquid of the drilling mud and the other drilling mud solids are discharged from separator 24 through line 28 as a dirty liquid stream substantially free of ferrous abrasives. The slurry is pumped by a suitable pump 30, which may be a Moyno pump, to a centrifugal cleaner 32 in which further' separation of the drilling mud and ferrous abrasive occurs. Centrifugal cleaner 32 is described and claimed in a patent application of Eber W. Gaylord entitled Centrifugal Cleaner filed concurrently herewith. In cleaner 32 the slurry is discharged into a guide at the center of a rotating impeller having a plurality of impeller blades inclined in a direction opposite the direction of rotation of the impeller. The slurry flows from the guide onto the inner end of the impeller blades. Abrasive particles are discharged from the trailing edge of the impeller blades and drilling mud from the outer end. Ferrous abrasive particles discharged from cleaner 32 are delivered-through line 34 to a conveyor 36 which transports the ferrous abrasive particles into the inlet end of rotary kiln drier 38.

The major part of the liquid delivered into cleaner 32 with the solids other than the ferrous abrasive particles and a small part of the ferrous abrasives, particularly broken abrasive particles, charged to the cleaner 32 are discharged through line 40 and pumped by a pump 42 through line 44 into a cyclone-separator 46. Ferrous abrasive particles are discharged from the lower end of the cyclone separator onto a screen 48 where the abrasive particles are washed by a water stream circulated by pump 50. It is necessary periodically to replace the water used in the washing because of the buildup in the concentration of nonferrous solid particles. Ferrous abrasive particles discharged from the screen 48 are delivered through line 51 onto conveyor 36 for delivery with the abrasive particles from separator 32 into the kiln drier 38.

The arrangement illustrated in the drawing for processing the liquid discharged from the cleaner is one of several that can be used. The processing of that liquid will depend on the amount of abrasive broken in the drilling operation because broken abrasive particles tend to remain in the drilling mud and a large part of them are discharged from the cleaner with the drilling mud. If only a small part of the abrasives are broken, the drilling mud can be returned through a line 43 to the magnetic separator 24. Drilling mud would then be bled from the system periodically through line 41 to prevent a buildup of broken abrasives in the system. Another alternative is to deliver the drilling mud discharged from the cleaner to a second magnetic separator to recover the small amount of abrasives not separated from the drilling mud in the cleaner.

Hot gases pass through the kiln drier 38 countercurrent to the ferrous abrasive particles to evaporate moisture from the ferrous abrasive particles. Hot gases circulated through the drier to provide the heat necessary to evaporate the water can be obtained from a suitable source such as a burner in the drier or the exhaust from the motors used to drive the high pressure pumps used to circulate the abrasive laden liquid down the well. Dry ferrous abrasive particles discharged from the kiln 38 are delivered through line 52 to an elevator 54 which lifts the shot particles and discharges them onto the upper surface of a screen 56. The oversized and undersized shot particles are rejected from the system through lines 58 and 60 respectively and the dried and sized shot particles are delivered through line 62 into a suitable storage hopper 61. Make-up abrasive is added to the system at 53 to replace abrasive removed from the system as fines.

Dirty liquid from the separator 24 is delivered through line 28 into compartment 62A of a mud pit 62 divided into compartments 62A, 62B, and 62C by suitable partitions. A pump 64 withdraws dirty liquid through line 66 from compartment 62A and delivers it into cyclone separators 68. The underflow from the cyclone separators, which contains cuttings and a small amount of abrasive particles, is discharged from the system through line 70. Clean liquid is discharged from cyclone separators 68 as an overhead stream and is delivered through line 72 into clean-liquid compartment 62B. The term clean liquid refers to the liquid component of the drilling mud including suspended clay solids but from which abrasive and cuttings have been removed. A pump 74 withdraws clean liquid from compartment 628 through line 76 and delivers a part of the clean liquid into line 78 to adjust the volume of liquid passing through line 44 for most efficient operation of cyclone separator 46. Another portion of the clean liquid is delivered through line 80 into compartment 62A to maintain a constant supply of liquid as needed for most efficient operation of cyclone separators 68. A third portion of the clean liquid is returned through line 81 into clean liquid compartment 623. Makeup chemicals are added to the stream in line 81 from a suitable hopper 82 as required to maintain the desired drilling mud properties.

Clean liquid in compartment 628 is withdrawn through a line 84 by a pump 86 and passed through a heat exchanger 88 to remove heat imparted to the drilling mud during the jet drilling operation. A portion of the cooled clean liquid is returned to compartment 628 through line 90 for recycling through the heat exchanger. Another portion of the clean liquid is discharged through line 92 onto a vibrating screen 94 to remove any oversized solid particles that might have fallen into the liquid. The cleaned, cooled, and screened liquid is delivered from screen 94 through line 96 into compartment 62C. Compartment 62C should be covered to prevent contamination of the cleaned and cooled liquid.

A pump 98 withdraws clean liquid from compartment 62C through line 100 and delivers it through lines 102 and 104 into a blender 106 equipped with mechanically driven paddles to obtain homogenity and to keep the solids suspended in the liquid. Cleaned, dried, and screened ferrous abrasive is discharged from hopper 61 into a weigh feeder 108 which delivers the ferrous abrasive at a controlled rate into the blender 106. A concentrated (l5 50 percent by bulk volume) suspension of ferrous shot in clean liquid is withdrawn from the blender 106 and delivered at a low pressure of about l00 lbs. per square inch by pump 110 to a high pressure injector 112. Pump 1l0is preferably a Moyno type pump. lnjector 112 is a plunger type pump with a long stroke and operating at a low number of strokes per minute to increase the pressure on the concentrated suspension of ferrous abrasive in clean drilling mud to 5,000 20,000 p.s.i. and discharges the suspension into a line 114 for delivery to the well 10. The major portion of the clean drilling mud in line 102 is delivered through line 116 to a battery of high pressure pumps, indicated generally by reference numeral 118, suitable for handling a clean liquid and discharging the liquid at a pressure of approximately 5,000 20,000 p.s.i. The clean liquid is delivered through line 120 into line 1 14 for mixing with the concentrated suspension of ferrous abrasive particles to form the drilling mud used in the abrasive jet drilling process. Drilling mud is returned to kelly 12 and then to well 10 through line 114.

In a typical apparatus utilizing this invention, magnetic separator 24 will separate substantially all, for example 99 percent, of the ferrous abrasives from the drilling mud delivered to the separator and discharge I through line 26 that ferrous abrasive in a slurry in separator 24 are normally discharged from the separator through line 28.

The cleaner 32 which utilizes large dynamic forces to multiply differences in mobility of the abrasive particles and drilling mud over the impeller blades to separate the ferrous abrasives from the drilling mud liquids and very fine solids discharges through line 34 about 90 95 percent of the abrasives charged to the cleaner in a stream containing 1 4 percent by weight of drilling mud liquids. Approximately 90 percent of the drilling mud liquids delivered to the cleaner 32 is discharged from the cleaner through line 40 and delivered into cyclone separators 46. The cyclone separators 46, which may be for example 4-inch cones, complete the removal of ferrous abrasives and about 50 percent of the rock cuttings from the drilling mud introduced into the cones. The ferrous abrasives discharged from the screen 48 may constitute about 6 7 percent of the abrasives originally charged to the magnetic separator and contain about 10 percent by weight drilling mud liquids. The amount of ferrous abrasives discharged from the screen 48 will depend upon the extent to which the abrasive is broken in the drilling operation and the operation of the cleaner 32.

The cyclone separators 68 which may be 6-inch cones remove substantially all ferrous abrasives from the dirty liquid charged to those separators and about 95 percent of the rock cuttings. Only very finely divided rock particles and clay solids remain in the clean liquid discharged overhead from separators 68.

It is essential to the efficient operation of the abrasive jet drilling process that solid particles, including broken abrasive particles, other than the ferrous abrasives of the desired particle size be removed from the drilling mud before the drilling mud is recycled through the well. While abrasive particles having a size of 50 mesh are preferred, that range may extend to particles as small as 80 mesh. Separation of broken abrasive particles and other solids smaller than 80 mesh from the unbroken abrasive particles by means of screens while the abrasive particles are suspended in drilling mud is not feasible because of the low capacity of screens for making that separation. If the abrasive particles are suspended in drilling mud of the type disclosed in U. S.'Pat. No. 3,508,621 which contains an appreciable quantity of plant fibers, the fibers in the drilling mud blind the screens and make separation by screening virtually impossible.

In the treatment of the drilling mud according to this invention, use is made of the differences in magnetic properties of the abrasive particles and other mud constituents and the differences in mobility of the abrasive particles to make the separation of abrasive particles from other drilling mud constituents. The magnetic separator, additionally, has the important advantage that drilling mud can merely drain from the shale shaker into the tank of the magnetic separator. In contrast, cyclone separators require pumps to deliver the drilling mud into the cyclone separator at a high velocity to make the desired separation, and such pumps are subjected to severe erosion. In the process for treating the drilling mud described herein, substantially all of the abrasive particles are removed from the drilling mud before the drilling mud must be pumped for delivery at a high velocity into a cyclone separator.

Drying of the ferrous abrasive particles, particularly if the ferrous abrasive is used with ,the drilling mud described in U. S. Pat. No. 3,508,621, is necessary to make an efficient separation of broken and unbroken ferrous abrasive particles; however, the ferrous abrasive discharged from the magnetic separator cannot be dried effectively in a kiln drier or fluidized bed type drier. Twenty-five, or even more, percent by weight of the abrasive laden slurry from the magnetic separator is drilling mud. If that slurry is charged directly to the drier, the abrasive particles are bonded together in the drier. It is necessary to reduce the amount of drilling mud charged to the drier with the abrasive to below about 5 percent by weight of abrasive particles. The centrifugal cleaner 32 removes approximately 90 percent of the drilling mud from the abrasive particles charged to the cleaner and discharges a stream containing as little as l 2 percent drilling mud for delivery to the drier. The clean abrasive particles discharged from the cleaner can be readily dried without clumping in either a kiln type or fluidized bed type drier. The reduced moisture content of the abrasive particles has an additional important advantage of reducing the heat required by the drier.

The dried abrasive particles can readily be separated from broken abrasive particles by screening. A five stack screen has been found to be advantageous in giving a compact structure of adequate capacity. Dried abrasives can be stored without corroding and are easily charged at an accurately controlled weight to a blender for mixing with clean drilling mud to form a suspension of uniform concentration of abrasive particles in drilling mud for recirculating to the well.

We claim:

1. A method of treating ferrous abrasive laden drilling mud discharged from a well for recirculation in an abrasive jet drilling process comprising screening oversize cuttings from the drilling mud, dividing the screened drilling mud into a ferrous-abrasive-rich stream and a drilling mud stream substantially devoid of ferrous abrasives, cleaning the ferrous abrasives in the ferrous-abrasive-rich stream to reduce the concentration of drilling mud with the ferrous abrasives to below about 5 percent by weight, drying the clean ferrous abrasives, and mixing the dried ferrous abrasive with drilling mud liquids at a controlled rate to form a drilling mud for recirculating through the well.

2. A method as set forth in claim 1 in which the division of the screened drilling mud is accomplished by magnetically separating ferrous abrasives from the drilling mud.

3. A method a set forth in claim 1 in which thedivision of the screened drilling mud is accomplished by magnetically separating ferrous abrasives from drilling mud and the reduction of the concentration of drilling mud on the ferrous abrasives is accomplished by centrifugally cleaning the drilling mud from the ferrous abrasive particles.

4. A method as set forth in claim 3 in which the dried ferrous abrasives are screened to remove particles larger than 20 mesh and particles smaller than mesh before mixing with drilling mud liquids.

5. A method as set forth in claim 3 in which drilling mud discharged from the centrifugal cleaning is passed through a cyclone separator to separate abrasive particles therefrom and the abrasive particles from the separator are dried with abrasive particles delivered from the centrifugal cleaner directly to the drier.

6. A method as set forth in claim 3 in which drilling mud discharged during centrifugal cleaning of the magnetically separated ferrous abrasive particles is recycled through the magnetic separator.

7. A method as set forth in claim 1 in which the stream of drilling mud substantially devoid of abrasive particles that was divided from the screened drilling mud is passed through a cyclone separator to remove fine particles therefrom and form a clean drilling mud, and the cleaned drilling mud is mixed with dried abrasive particles.

8. A method as set forth in claim 7 in which the cleaned drilling mud is cooled and screened, and the cooled and screened drilling mud is mixed with the dried ferrous abrasive particles to form the abrasive laden drilling mud for recirculating to the well.

9. A method of treating ferrous abrasive laden drilling mud discharged from a well for recirculation in an abrasive jet drilling process comprising screening oversize cuttings from the drilling mud, magnetically separating ferrous abrasives from the screened drilling mud, cleaning the magnetically separated ferrous abrasives to reduce the concentration of drilling mud thereon to less than percent by weight of the ferrous abrasives, drying the cleaned ferrous abrasives, screening the dried ferrous abrasives to remove oversize and undersize particles, passing drilling mud from the magnetic separation through a cyclone separator to remove solid particles therefrom and form a clean drilling liquid, cooling the clean drilling liquid, blending screened dried ferrous abrasives with a first portion of the cleaned drilling liquid to form a slurry containing 50 percent abrasive particles, increasing the pressure on the slurry to 5,000 20,000 p.s.i., pumping a second portion of the clean drilling liquid to a pressure of 5,000 20,000 p.s.i., mixing the clean drilling liquid and the slurry at a pressure of 5,000 20,000 p.s.i. to form a drilling mud for recirculating to the well.

10. Apparatus for treating ferrous abrasive laden drilling mud discharged from a well in an abrasive jet drilling process to form an abrasive laden drilling mud for recirculation through the well comprising means for separating abrasive particles from the drilling mud discharged from the well, a cleaner for removing drilling mud from the separated abrasive particles, means for transporting the ferrous abrasive particles to the cleaner, a drier for drying the clean abrasive particles, means for delivering the cleaned abrasive particles to the drier, and means for mixing dried abrasive particles at a controlled rate into drilling mud liquids to form an abrasive laden drilling mud for recirculating to thewell.

11. Apparatus as set forth in claim 10 in which the means for separating is a magnetic separator.

12. Apparatus as set forth in claim 10 in which the means for removing drilling mud from the separated abrasive particles is a centrifugal cleaner.

13. Apparatus for reconditioning drilling mud discharged from a well in the abrasive jet drilling process for recirculation through the well comprising a screen for separating oversize cuttings from the drilling mud discharged from the well, a magnetic separator, means for delivering screened drilling mud from the screen to the magnetic separator, a centrifugal cleaner for removal of drilling mud from the ferrous abrasive particles, means for delivering ferrous abrasive particles from the magnetic separator to the centrifugal 

1. A method of treating ferrous abrasive laden drilling mud discharged from a well for recirculation in an abrasive jet drilling process comprising screening oversize cuttings from the drilling mud, dividing the screened drilling mud into a ferrous-abrasive-rich stream and a drilling mud stream substantially devoid of ferrous abrasives, cleaning the ferrous abrasives in the ferrous-abrasive-rich stream to reduce the concentration of drilling mud with the ferrous abrasives to below about 5 percent by weight, drying the clean ferrous abrasives, and mixing the dried ferrous abrasive with drilling mud liquids at a controlled rate to form a drilling mud for recirculating through the well.
 2. A method as set forth in claim 1 in which the division of the screened drilling mud is accomplished by magnetically separating ferrous abrasives from the drilling mud.
 3. A method a set forth in claim 1 in which the division of the screened drilling mud is accomplished by magnetically separating ferrous abrasives from drilling mud and the reduction of the concentration of drilling mud on the ferrous abrasives is accomplished by centrifugally cleaning the drilling mud from the ferrous abrasive particles.
 4. A method as set forth in claim 3 in which the dried ferrous abrasives are screened to remove particles larger than 20 mesh and particles smaller than 80 mesh before mixing with drilling mud liquids.
 5. A method as set forth in claim 3 in which drilling mud discharged from the centrifugal cleaning is passed through a cyclone separator to separate abrasive particles therefrom and the abrasive particles from the separator are dried with abrasive particles delivered from the centrifugal cleaner directly to the drier.
 6. A method as set forth in claim 3 in which drilling mud discharged during centrifugal cleaning of the magnetically separated ferrous abrasive particles is recycled through the magnetic separator.
 7. A method as set forth in claim 1 in which the stream of drilling mud substantially devoid of abrasive particles that was divided from the screened drilling mud is passed through a cyclone separator to remove fine particles therefrom and form a clean drilling mud, and the cleaned drilling mud is mixed with dried abrasive particles.
 8. A method as set forth in claim 7 in which the cleaned drilling mud is cooled and screened, and the cooled and screened drilling mud is mixed with the dried ferrous abrasive particles to form the abrasive laden drilling mud for recirculating to the well.
 9. A method of treating ferrous abrasive laden drilling mud discharged from a well for recirculation in an abrasive jet drilling process comprising screening oversize cuttings from the drilling mud, magnetically separating ferrous abrasives from the screened drilling mud, cleaning the magnetically separated ferrous abrasives to reduce the concentration of drilling mud thereon to less than 5 percent by weight of the ferrous abrasives, drying the cleaned ferrous abrasives, screening the dried ferrous abrasives to remove oversize and undersize particles, passing drilling mud from the magnetic separation through a cyclone separator to remove solid particles therefrom and form a clean drilling liquid, cooling the clean drilling liquid, blending screened dried ferrous abrasives with a first portion of the cleaned drilling liquid to form a slurry containing 15 - 50 percent abrasive particles, increasing the pressure on the slurry to 5,000 - 20,000 p.s.i., pumping a second portion of the clean drilling liquid to a pressure of 5,000 - 20, 000 p.s.i., mixing the clean drilling liquid and the slurry at a pressure of 5,000 - 20,000 p.s.i. to form a drilling mud for recirculating to the well.
 10. Apparatus for treating ferrous abrasive laden drilling mud discharged from a well in an abrasive jet drilling process to form an abrasive laden drilling mud for recirculation through the well comprising means for separating abrasive particles from the drilling mud discharged from the well, a cleaner for removing drilling mud from the separated abrasive particles, means for transporting the ferrous abrasive particles to the cleaner, a drier for drying the clean abrasive particles, means for delivering the cleaned abrasive particles to the drier, and means for mixing dried abrasive particles at a controlled rate into drilling mud liquids to form an abrasive laden drilling mud for recirculating to the well.
 11. Apparatus as set forth in claim 10 in which the means for separating is a magnetic separator.
 12. Apparatus as set forth in claim 10 in which the means for removing drilling mud from the separated abrasive particles is a centrifugal cleaner. 